Double-tuned circuit

ABSTRACT

A double-tuned circuit in which a capacitor is connected to offset the effect of internal tube or transistor reactances and permit up to unity mutual coupling in an interstage circuit. In a preferred embodiment, a metallic cylinder positioned around the tube is used as the external cathode lead to lower its inductance.

United States Patent [72] Inventor Donovan C. Davis Pasadena, Calif.[21] Appl. No. 650,769 [22] Filed July 3, 1967 [45] Patented June 1,1971[73] Assignee Hoffman Electronics Corporation El Monte, Calif.

[54] DOUBLE-TUNED CIRCUIT 14 Claims, 6 Drawing Figs.

[52] U.S.C1 330/178, 330/15, 333/76 [51] Int. Cl 1103f 1/42, H031 H44[50] Field of Search 330/154, 65,66,176,67,165,166;333/70,178,61/76 [56]References Cited UNITED STATES PATENTS 2,235,003 3/1941 Arends .1 333/70Primary Examiner-Nathan Kaufman A!wrney-Lyon and Lyon ABSTRACT: Adouble-tuned circuit in which a capacitor is connected to offset theeffect of internal tube or transistor rcactances and permit up to unitymutual coupling in an interstage circuit. In a preferred embodiment, ametallic cylinder positioned around the tube is used as the externalcathode lead to lower its inductance.

' I l l I DOUBLE-TUNED CIRCUIT BACKGROUND OF THE INVENTION Thewell-known double-tuned circuit is often used to couple the stages of amultistage amplifier. Of particular significance in such double-tunedcircuits are critical coupling and transitional coupling. Criticalcoupling may be defined as the degree of mutual coupling between theprimary and secondary circuits at which maximum power transfer isachieved, that is, where the primary circuit resistance is transformedto equal the secondary circuit resistance. If the coefficient ofcoupling is increased beyond critical, transitional coupling may beachieved. At transitional coupling, the maximum flatness of the passbandof the double-tuned circuit is achieved. For any coupling greater thantransitional, a double humped response will occur, while for anycoupling less than transitional, a single response of narrower bandwidth is obtained. However, maximum power transfer is not obtained attransitional coupling except in a special case of equal primary andsecondary when transitional coupling and critical coupling are equal.

In the design of wide band amplifiers, it is frequently desirable toobtain the maximum possible bandwidth in each in terstage circuit aslimited by parameters of the active amplifier elements which haveinherent input and output resistances and series and shunt reactances.For example, in a wideband vacuum tube amplifier operated in the VHF orUHF frequency regions the lead inductances, shunt impedances and inputand output resistances are critical to the achievement of maxin'iumbandwidth and power transfer. The significant parameters of the tubesthemselves are the cathode and plate series inductances and the cathodegrid and grid-plate capacitances. At frequencies above several hundredmegahertz, the lead inductance internal to the tube envelope isappreciable in even the best tubes designed for this purpose. Inaddition, the capacitive reactance from cathode to grid and from plateto grid become very low compared to the input and output resistances ofthe tube at these higher frequencies thereby increasing the Q of theinput and output circuits and making the achievement of broad bandwidthsdifficult or impossible unless external loading decreases the power gainof the stage more than the bandwidth is increased, resulting in adecreased power gain bandwidth product. Consequently, external loadingis an undesirable and frequently unacceptable means of obtaining broadbandwidth in an RF power amplifier.

In the double-tuned circuit it is necessary that the primary and secondcircuits each individually be turned to a frequency at or near thecenter of the desired band-pass of the circuit. This "requires aspecific relationship between the total capacitance and inductance ofboth the primary and secondary circuits, as the total inductance whichcan be used in either primary or secbndary circuit to resonate that halfof the circuit is determined by the total shunt capacity. As aconsequence of this fact and because of the inductance present in thecathode and plate leads of the tube internal to the tube envelope, thetotal amount of mutual inductance which can be employed is limited inthe conventional double-tuned circuit since the mutual inductance is ofnecessity external to the tube itself. However, because of themagnitudes of the internal reactances of the tubes, and the normal loadsto be encountered, the Os of the primary and secondary circuits may besuch to require a coefficient of coupling which can be achieved only byusing a mutual inductance larger than that which permits secondarycircuit resonance to be achieved. As a consequence transitional couplingcannot be achieved. 7

This problem is aggravated because of the relatively high inductance ofthose connectors, such as straps or bars, which are conventionally usedas the external cathode lead. Large reactances in the coupling circuitreduce the network bandwidth and thus decrease the performance of thecircuit.

SUMMARY OF THE INVENTION According to the present invention, sufficientcoupling to achieve transitional or even overcoupling can be obtained bythe addition of an external series capacitive reactance in the secondarycircuit to cancel the inductive reactance of the internal cathode leadat the center frequency of the amplifier. This effective reduction insecondary circuit inductive reactance permits the use of a large enoughmutual inductance to permit transitional coupling. In order to make thebandwidth as broad as possible, it is desirable to reduce the externalcathode lead to as small an inductance as possible, thus also permittingthe use of a smaller compensating capacitor. This is accomplishedaccording to the present invention by providing this cathode leadinductance in the form of a cylinder which surrounds the tube and whichserves as the internal conductor of a coaxial cable, the outer conductorof which is the housing in which the circuitry is located. Theinductance of the cylinder is thus a function of the diameter of thecylinder and itsspacing from the outer wall.

Although the external capacitor inserted in the secondary produces zeronet reactance only at the center frequency, at a frequency :10 percentfrom band center, the net inductive or capacitance reactance of thecapacitance and inductance in series is only 20 percent of the originalreactance of the cathode inductance. This is still small enough topermit transi tional or greater than transitional coupling to beachieved in the interstage circuit. This permits bandwidths of 20percent- 30 percent of center frequency to be achieved with transitionalcoupling by the simple expedient of resonating the cathode leadinductance internal to the tube by an external se ries capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of aconventional doubletuned circuit interconnecting two amplifier tubes;

FIG. 2 is a schematic diagram of the circuit of the present invention;

I FIG. 3 is a top plan view of a structural embodiment of theDESCRIPTION OF THE INVENTION The problems overcome by the presentinvention and the improved operation resulting therefrom may be betterunderstood by discussion of a typical example of a prior art circuitsuch as that shown in FIG. I. This circuit will be recognized as adouble-tuned circuit connecting two stages of an amplifier with k theinternal reactances of the tubes and the external reactances beingillustrated. As shown, tubes 10 and 11 are connected as grounded gridamplifiers and are suitable for use in the UHF and higher frequencyregions. These tubes, for example, may be Type 791 l manufactured by theGeneral Electric Company. The grid to plate capacitance C of the tube 10is illustrated by the capacitor 12 while the internal plate leadinductance L of the tube 10 is illustrated by the inductance 13. In vasimilar manner, the cathode to grid capacitance C of the tube 11 isillustrated by the capacitor 14 while the internal cathode leadinductance L is illustrated by the inductor 15. As shown, the tubecircuits are interconnected by the T equivalent of a double-tunedcircuit comprising series inductors 16 and I7, and the shunted mutualinductance (L 18. A blocking capacitor I9 is provided to prevent the DCon the plate of the tube 10 from being grounded through the inductor I8.

Let us now analyze the circuit shown in FIG. I, keeping in mind thedesirability of operating the circuit at transitional coupling. Assumingthat the amplifier is operating at I000 MegaI-Iertz with the groundedgrid triodcs I0 and II having a shunt input capacitive reactance of j 25ohms and a cathode lead inductive reactance internal to the tube of +j 8ohms. Assume the external cathode lead reactance to be +j 10 ohms. Sincethe total inductive reactance must equal the total capacitive reactanceat the center frequency, the maximum mutual inductance (L is +j 7 ohms.A representative load resistance presentrd by the grid-cathode tubeinput at these frequencies would be approximately 30 ohms resulting in asecondary circuit Q of 1.2 (R /C =30/25=l .2).

Representative values of the output impedance of the tube would ber,=2,000 ohms, the plate to grid capacitance ll of the tube j 60 ohms,and the internal plate lead inductance l3 ofthe tube +j 30 ohms. Theblocking capacitor 19 would have a reactance of j ohms and the externalinductance 16 a reactance of +j 33 ohms. As a consequence, the Q of theprimary circuit, that is, r,,/X ,,=2,000/60=33.33. By reference to theliterature, for example, Electronir Designers-r Handbook by Landee,Davis and Albrecht, FIG. 13.21, it can be found that to achievetransitional coupling with a secondary Q of 1.2 and a ratio ofQ primaryto Q secondary of 27.78, the required coefficient of coupling isapproximately 0.45. Since the total primary circuit inductive reactancemust equal +j 75 ohms and the total secondary inductive reactance mustequal +j 25 ohms, the required mutual inductance to achieve transitionalcoupling is found from the equation:

Where:

M=mutual inductance between primary and secondary,

k=coefficient of coupling.

L,,=primary circuit inductance.

L,=secondary circuit inductance.

The required mutual inductance is thus found to be 19.5 ohms. However,because the internal cathode lead inductance 15 of the tube 11 is +j 8ohms and the external cathode lead inductance 17 is +j 10 ohms,secondary circuit resonance cannot be achieved with a mutual inductancewhich is larger than +j 7 ohms. As a consequence, transitional couplingcannot be achieved.

FIG. 2 shows the circuit of FIG. 1 modified in accordance with thepresent invention. The circuit shown in FIG. 2 is almost identical withthat shown in FIG. 1, and similar reference numerals are used todesignate similar elements. As can be seen, a capacitor has been addedin the external portion of the secondary circuit of FIG. 2. The value ofthis capacitor is chosen so that the secondary circuit can be made toresonate even though the value of the mutual inductance 18 is selectedin accordance with criterion other than simply those of the secondarycircuit. Thus, in the example given previously, the capacitor 20 wouldhave a capacitive reactance of j 12.5 ohms. This permits the mutualinductance 18 to be increased by l2.5 ohms, that is, up to the 19.5 ohmlevel that is required. Consequently, transitional coupling is achievedand the greatest possible power gain-bandwidth product is obtained.

Turning now to FIGS. 3, 4, 5 and 6, there is shown a structuralembodiment of the circuit shown in FIG. 2. A conductive housing 24 ofaluminum or the like which serves as circuit ground is mounted on anassembly 25 which may contain other portions of the electrical systemwith which the amplifier is used. The housing 24 is provided with agroove 26 on its upper surface which receives an O-ring 27 whichcooperates with a cover plate 28 to seal a chamber 29 within the housing24 from moisture or other contaminants.

The housing 24 is provided with a series of lips 30 that protrude intothe chamber 29. Each pair of lips 30 cooperate with a clamp 31 tosupport a vacuum tube 32 and to make contact to the grid thereof. As canbest be seen in FIG. 6, the clamp 31 consists ofa pair of plates 33 and34 whose ends are slightly hooked so as to form clamps for grasping thelips 30. The plates 33 and 34 are positioned on either side of the gridcontact area 35 of the tube 32 and then spot-welded or otherwisefastened together so as to hold the tube firmly in place. The tube 32,as previously mentioned, is preferably a General Electric Type 791 I.Contact to the cathode contact area 36 of the tube is made by a cylinder37 that is positioned over the cathode end of the tube. The bottom plate38 of the cylinder 37 serves as one plate of a capacitor, the otherplate of which is a bracket 39 connected to one side of the housing 24and the dielectric of which is one or more sheets 40 of a suitablematerial such as polytetrafluoroethylenc sold by DuPont under the tradename Teflon.

A capacitor 41 is connected to the bracket 39 between its ends and is,in turn, connected through a sliding bar type variable inductance 42 tothe anode pin 43 of the next tube. This sequence of connections isrepeated as many times as desired. If desired, the cylinder 37 can beprovided with a slot 43 to permit access to the tube of the variouschokes which are conventionally used with such tubes. If desired, a slotcan be provided on both sides of the cylinder but it is preferable thatthe slots not extend the whole length of the cylinder so that aconnection remains between the upper and lower halves thereof. Ifdesired, the cylinder 37 can be made shorter and the capacitor formed ona surface thereof by positioning the dielectric and the other plate in asaddlelike manner on the surface.

Each of the electrical elements shown in FIG. 2 finds its structuralmanifestation in FIG. 3. The tubes 10 and 11 correspond to thesuccessive tubes 32 in FIG. 3. The capacitor 12 and inductor l3 andcapacitor 14 and inductor 15 are internal impedances of these tubes aspointed out previously. The inductor 16 of FIG. 2 corresponds to thevariable inductor 42 of FIG. 3. The capacitor 19 of FIG. 2 correspondsto the capacitor 41 of FIG. 3. The mutual inductance 18 of FIG. 2corresponds to that portion of the bracket 39 from the capacitor 41 tothe housing 24, that is, to ground. The capacitor 20 is made up of thebracket 39, dielectric 40 and plate 38 of FIG. 3. The inductor 17 ofFIG. 2 corresponds to the cylinder 37 of FIG. 3.

The use of the cylinder 37 as the external cathode lead inductance ofthe tube permits this inductance to be substantially reduced over otherinductances commonly used for this purpose. The cylinder acts as theinner conductor of a coaxial cable, the housing 24 acting as the outerconductor thereof. The inductance the cylinder presents is a function ofits diameter and its spacing from the outer wall of the housing 24, orin this case, the base of the housing 24 and the cover plate 28 sincethey are the closest surfaces of the housing. The actual impedance ofthe cylinder and its inductive reactance can be determined byconventional mathematics and need not be discussed here.

From the foregoing description, it can be seen that a double-tunedcircuit has been provided which permits the desired condition oftransitional coupling to be achieved even when the circuit is operatingat frequencies high enough to require that the internal reactances ofvacuum tubes be taken into account as circuit components. A uniquestructural configuration has also been provided which permits theexternal cathode lead inductance of the tubes utilized to be reducedtherefore permitting the value of the compensating capacitor to bereduced and the bandwidth of the system increased.

The invention may be embodied in other specific forms not departing fromthe spirit or central characteristics thereof. The present embodimentsare therefore to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing description, and all changes whichcome within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

' Iclaim:

1. In a circuit for coupling a pair of amplifying means at frequenciesat which the internal reactances of the amplifying means becomessignificant circuit parameters and including a double-tuned circuithaving a primary circuit, a secondary circuit and a mutual inductivereactance common to said primary and secondary circuits; said primarycircuit including as reactive components the internal inductive andcapacitive reactances of the first of said amplifying means and theinductive reactance of an output connector coupled to said firstamplifying means; said secondary circuit including as reactivecomponents the internal inductive and capacitive reaetance of the secondof said amplifying means and the inductive reaetance of an inputconnector coupled to said second amplifying means; the improvementcomprising an external capacitive reaetance connected in said secondarycircuit, the value of said external capacitive reaetance being selectedto offset a portion of the inductive reaetance of said secondary circuitto permit said mutual inductive reaetance to be chosen to obtaintransitional coupling between said primary and secondary circuits whilemaintaining secondary circuit resonance.

2. In a circuit for coupling a pair of amplifying means each having aninput electrode, an output electrode and a control electrode at afrequency at which the internal reactances of the amplifying meansbecome significant circuit parameters and including a double-tunedcircuit having a primary circuit, a secondary circuit and a mutualinductive reaetance common to said primary and secondary circuits; saidprimary circuit including as reactive components the controlelectrode-output electrode capacitive reaetance and the output electrodeinductive reaetance of the first of said amplifying means and theinductive reaetance of an output connector coupled to the outputelectrode of said first amplifying means; said secondary circuitincluding as reactive components the input electrodecontrol electrodecapacitive reaetance and the input electrode inductive reaetance of thesecond of said amplifying means and the inductive reaetance of an inputconnector coupled to the input electrode of said second amplifyingmeans; the improvement comprising an external capacitive reaetancecoupled in said secondary circuit in series with said input connectorinductive reaetance, the value of said external capacitive reaetancebeing selected to offset a portion of the inductive reaetance of saidsecondary circuit to permit said mutual inductive reactance to be chosento obtain transitional coupling of said frequency between said primaryand secondary circuits while maintaining secondary circuit resonance.

3. The circuit of claim 2 wherein said control electrodes of saidamplifying means are grounded.

4. The circuit of claim 2 wherein said amplifying means comprise spacedischarge devices each having a cathode, an anode and a grid.

5. The circuit of claim 2 wherein said primary circuit includes ablocking capacitor.

6. The circuit of claim 2 wherein said input connector comprises oneconductor of a coaxial cable.

7. The circuit of claim 6 wherein said input connector comprises acylinder positioned around said second amplifying means.

8. The circuit of claim 7 wherein the other conductor of said coaxialcable comprises a housing in which said amplifying means are mounted.

9. The circuit of claim 8 wherein said external capacitive reaetancecomprises a portion of said cylinder, dielectric means positionedthereaginst, and bracket means positioned against said dielectric meansand connected to said housing.

10. The circuit of claim 9 wherein said portion of said cylindercomprises a base plate fastened thereto.

11. An amplifying circuit for use at frequencies at which the internalreactances of amplifying means become significant circuit parameterscomprising: first and second amplifying means each having an inputelectrode, an output electrode and a control electrode; a closedconductive housing for enclosing said amplifying means; clamp means forsupporting said amplifying means in said housing and electricallyconnecting the control electrodes ofsaid amplifying means to saidhousing; inductive reaetance means coupled to said output electrode ofsaid first amplifying means; a conductive cylinder positioned around aportion of said second amplifying means and electrically connected tothe input electrode thereof; bracket means mechanically and electricallyconnected to said housing and extending into proximity with a portion ofsaid cylinder; dielectric means separating said bracket means and saidcylinder portion whereby a capacitive reaetance is formed; andcapacitive reaetance means electrically coupling said bracket means at apoint intermediate the ends thereof with said inductive reaetance means;said named reactances and said internal rcactances of said amplifyingmeans forming a double-tuned circuit with the portion of said bracketmeans from said point intermediate the ends thereof to said housingbeing the mutual inductance of said double-tuned circuit, saidcapacitive reaetance formed by said bracket means, said cylinder andsaid dielectric means permitting transitional coupling in saiddouble-tuned circuit at said frequencies.

12. The circuit of claim 1] wherein said amplifying means comprise spacedischarge devices each having a cathode, an anode and a grid.

. 13, The circuit ofclaim ll wherein said inductive reaetance meanscomprises a sliding bar variable inductance.

14. The circuit of claim 11 wherein said portion of said cylindercomprises a base plate fastened thereto.

1. In a circuit for coupling a pair of amplifying means at frequenciesat which the internal reactances of the amplifying means becomessignificant circuit parameters and including a double-tuned circuithaving a primary circuit, a secondary circuit and a mutual inductivereactance common to said primary and secondary circuits; said primarycircuit including as reactive components the internal inductive andcapacitive reactances of the first of said amplifying means and theinductive reactance of an output connector coupled to said firstamplifying means; said secondary circuit including as reactivecomponents the internal inductive and capacitive reactance of the secondof said amplifying means and the inductive reactance of an inputconnector coupled to said second amplifying means; the improvementcomprising an external capacitive reactance connected in said secondarycircuit, the value of said external capacitive reactance being selectedto offset a portion of the inductive reactance of said secondary circuitto permit said mutual inductive reactance to be chosen to obtaintransitional coupling between said primary and secondary circuits whilemaintaining secondary circuit resonance.
 2. In a circuit for coupling apair of amplifying means each having an input electrode, an outputelectrode and a control electrode at a frequency at which the internalreactances of the amplifying means become significant circuit parametersand including a double-tuned circuit having a primary circuit, asecondary circuit and a mutual inductive reactance common to saidprimary and secondary circuits; said primary circuit including asreactive components the control electrode-output electrode capacitivereactance and the output electrode inductive reactance of the first ofsaid amplifying means and the inductive reactance of an output connectorcoupled to the output electrode of said first amplifying means; saidsecondary circuit including as reactive components the inputelectrode-control electrode capacitive reactance and the input electrodeinductive reactance of the second of said amplifying means and theinductive reactance of an input connector coupled to the input electrodeof said second amplifying means; the improvement comprising an externalcapacitive reactance coupled in said secondary circuit in series withsaid input connector inductive reactance, the value of said externalcapacitive reactance being selected to offset a portioN of the inductivereactance of said secondary circuit to permit said mutual inductivereactance to be chosen to obtain transitional coupling of said frequencybetween said primary and secondary circuits while maintaining secondarycircuit resonance.
 3. The circuit of claim 2 wherein said controlelectrodes of said amplifying means are grounded.
 4. The circuit ofclaim 2 wherein said amplifying means comprise space discharge deviceseach having a cathode, an anode and a grid.
 5. The circuit of claim 2wherein said primary circuit includes a blocking capacitor.
 6. Thecircuit of claim 2 wherein said input connector comprises one conductorof a coaxial cable.
 7. The circuit of claim 6 wherein said inputconnector comprises a cylinder positioned around said second amplifyingmeans.
 8. The circuit of claim 7 wherein the other conductor of saidcoaxial cable comprises a housing in which said amplifying means aremounted.
 9. The circuit of claim 8 wherein said external capacitivereactance comprises a portion of said cylinder, dielectric meanspositioned thereaginst, and bracket means positioned against saiddielectric means and connected to said housing.
 10. The circuit of claim9 wherein said portion of said cylinder comprises a base plate fastenedthereto.
 11. An amplifying circuit for use at frequencies at which theinternal reactances of amplifying means become significant circuitparameters comprising: first and second amplifying means each having aninput electrode, an output electrode and a control electrode; a closedconductive housing for enclosing said amplifying means; clamp means forsupporting said amplifying means in said housing and electricallyconnecting the control electrodes of said amplifying means to saidhousing; inductive reactance means coupled to said output electrode ofsaid first amplifying means; a conductive cylinder positioned around aportion of said second amplifying means and electrically connected tothe input electrode thereof; bracket means mechanically and electricallyconnected to said housing and extending into proximity with a portion ofsaid cylinder; dielectric means separating said bracket means and saidcylinder portion whereby a capacitive reactance is formed; andcapacitive reactance means electrically coupling said bracket means at apoint intermediate the ends thereof with said inductive reactance means;said named reactances and said internal reactances of said amplifyingmeans forming a double-tuned circuit with the portion of said bracketmeans from said point intermediate the ends thereof to said housingbeing the mutual inductance of said double-tuned circuit, saidcapacitive reactance formed by said bracket means, said cylinder andsaid dielectric means permitting transitional coupling in saiddouble-tuned circuit at said frequencies.
 12. The circuit of claim 11wherein said amplifying means comprise space discharge devices eachhaving a cathode, an anode and a grid.
 13. The circuit of claim 11wherein said inductive reactance means comprises a sliding bar variableinductance.
 14. The circuit of claim 11 wherein said portion of saidcylinder comprises a base plate fastened thereto.